The present invention relates to AC protective circuits, and more particularly to electronic circuits for protecting the input of a radio receiver from excessive AC voltages.
As is known in the art, various radio receivers are subject to damage upon the receipt of excessive levels of RF input unless appropriate steps are taken to protect the receiver input from voltages above a predetermined level. Such excessive voltages may occur when a receiver is in close proximity to transmitters or upon the inadvertent malfunction of devices which may couple RF power to a receiver. In any event, the occurrence of such excessive energy will damage the components of the RF receiver, thereby creating additional failures in the communications system.
In an effort to protect receivers from such excessive voltages, a variety of techniques have been employed. In one example, clamping of the RF input is employed to hold the input voltage level to below a predetermined value so that excessive voltages cannot be generated which would be harmful to the receiver. In other instances, the input voltage to the receiver is monitored for the purpose of switching the RF input from the receiver upon the occurrence of an excessive voltage condition. Each technique involves the use of fairly complex circuitry, or at a minimum requires the dissipation of significant power even before the overload condition occurs.
In one specific technique as exemplified by U.S. Pat. No. 3,988,640, there is shown a typical circuit for monitoring the input voltage and clamping the overload upon the occurrence of an overload condition. In the particular technique, however, there is a positive dissipation of power required to maintain the operation of the overvoltage protection circuit, even during the period of time an overload does not exist. Accordingly, although the problem of overload protection is addressed, it is accomplished with an accompanying continuous power loss of some sort. There is therefore a continuing need to provide improved overload protection with less cost, less complexity, and which can be easily implemented in a variety of electronic circuits.
The present invention has therefore been developed to overcome the specific shortcomings of the above known and similar techniques and to provide an improved AC protection circuit.